I have never used a swing but I would think 20' on any 18x20 racket with a drop weight stringer would be difficult with a starting clamp. Most of the time it takes a little more string to reach the tension gripper with a drop weight than other types of tensioners. You may want to get yourself a starting clamp and bridge the string.
I guess you could mount the frame so the end you finish your mains on is closer to the pivot point of the turntable so the string would not need to be as long. If I have made a bad assumption and you have another type of trouble let me know.
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